1. Field of the Invention
The present invention relates to the production of supported catalysts, more specifically to the method for making supported catalysts containing nanometer sized particles comprised of different metals dispersed throughout the catalyst support material.
2. Description of the Related Art
Many industrial products such as fuels, lubricants, polymers, fibers, drugs, and other chemicals would not be manufacturable without the use of catalysts. Catalysts are also essential for the reduction of pollutants, particularly air pollutants created during the production of energy and by automobiles. The majority of industrial catalysts are composed of a high surface area support material upon which chemically active metal nanoparticles (i.e., nanometer sized metal particles) are dispersed. The support materials are generally inert, ceramic type materials having surface areas on the order of hundreds of square meters/gram. This high specific surface area usually requires a complex internal pore system. The metal nanoparticles are deposited on the support and dispersed throughout this internal pore system, and are generally between 1 and 100 nanometers in size.
Processes for making supported catalysts go back many years. One such process for making platinum catalysts, for example, involves the contacting of a support material such as alumina with a metal salt solution such as chloroplatinate acid. The salt solution “impregnates” or fills the pores of the support during this process. Following the impregnation, the support containing the salt solution would be air dried, causing the metal salt to precipitate within the pores. The support containing the crystallized metal salt would then be exposed to a hydrogen or carbon monoxide gas environment, reducing the solid metal salt to metal particles. The currently used methods for producing supported catalysts, including impregnation methods, are further discussed below.
Yoo et al., in an article entitled “Propene Hydrogenation Over Truncated Octahedral Pt Nanoparticles Supported on Alumina,” Journal of Catalysis, Vol. 214, pp. 1-7 (2003), disclose a process for loading colloidal platinum (Pt) nanoparticles (synthesized by a 1:5 concentration ratio of K2PtCl4 to polyacrylate capping polymer) into an alumina support via impregnation.
Miyazaki et al., in an article entitled “Morphology Control of Platinum Nanoparticles and Their Catalytic Properties,” Journal of Nanoparticle Research, Vol. 5, pp. 69-80 (2003), disclose the preparation of Pt nanoparticles of varying morphology through the use of different capping polymers. Various shapes (such as square, triangular, and hexagonal) of Pt crystallites, as observed by transmission electron microscopy (TEM), were obtained. Supported catalysts were made by impregnation of previously formed Pt crystallites into an alumina support. Water was removed from the support by freeze drying, and the capping polymers were removed by calcinating in air at 500° C. for 8 hours.
U.S. Pat. No. 6,569,358 discloses a method of preparing a porous material incorporating ultrafine metal particles comprising the following steps: (1) preparing surface-protected ultrafine metal particles by reducing metal ions in the presence of molecules such as dodecanethiol molecules; (2) immersing a wet gel in a solution of the ultrafine metal particles, thus forming an ultrafine metal particle/wet gel composite in which the ultrafine metal particles are incorporated in the wet gel; and (3) drying the ultrafine metal particle/wet gel composite to form a porous body.
The process disclosed in U.S. Pat. No. 6,569,358 utilizes protecting agents or capping polymers. However, removal of protecting agents or capping polymers can be an issue for sensitive catalytic processes, as their destruction may leave contaminating residues that are undesirable. These residues may reduce activity of the catalyst by occupying active sites necessary for subsequent reactions. The residues may also leave behind trace quantities of poisons that will eventually kill the catalyst over time. Removal of organic capping agents and polymers usually require oxidation (or burning), but the heat required for such oxidation may produce unwanted sintering due to the high temperatures. Sintering will increase the metal particle size and reduce the active surface area of the catalyst. Furthermore, the use of capping agents can hinder the introduction of the metal crystallites into small pores of the support.
U.S. Pat. No. 6,686,308 discloses a supported catalyst comprising catalyst metal nanoparticles having an average particle size of typically 2.0 nm or less, which are supported on support materials at a loading of 30% or more. It teaches the use of platinum, palladium, ruthenium, rhodium, iridium, osmium, molybdenum, tungsten, iron, nickel or tin, as catalyst metals, and the use of carbon as the support material. The method of making a supported catalyst disclosed in U.S. Pat. No. 6,686,308 comprises the steps of: (1) providing a solution of metal chlorides of one or more catalyst metals in a solvent system containing at least one polyalcohol, typically ethylene glycol containing less than 2% water; (2) forming a colloidal suspension of unprotected catalyst metal nanoparticles by raising the pH of the solution, typically to a pH of 10 or higher, and heating the solution, typically to 125° C. or higher; (3) adding support particles to the colloidal suspension; and (4) depositing the unprotected catalyst metal nanoparticles on the support particles by lowering the pH of the solution, typically to a pH of 6.5 or lower.
U.S. Pat. No. 6,603,038 discloses a method for producing a catalyst containing one or several metals from the group of metals comprising the sub-groups Ib and VIIIb of the periodic table on porous support materials, characterized by a first step, in which one or several precursors from the group of metal compounds from sub-groups Ib and VIIIb of the periodic table is or are applied to a porous support, and a second step in which the nanoporous support to which at least one precursor has been applied, is treated with at least one reduction agent to obtain the metal nanoparticles produced in situ in the pores of the support. In the first step, catalysts were prepared by impregnation of the support with a metal salt solution, followed by a drying step. Subsequent to drying, as part of the second step, the impregnated support materials were reduced by various techniques including re-impregnation with liquid reducing agents.